1. Field of the Invention
The present invention relates to improvement in a coil device for use in a flyback transformer, a switching power transformer, a choke coil or the like. And more particularly, it relates to improvements in a coil device employing a magnetic core with a gap.
2. Description of the Prior Art
In any of the conventional transformers, choke coils and so forth known heretofore, it is customary to form a gap in a closed magnetic path so that the magnetic core thereof is not saturated when a desired current is caused to flow. For example, when a ferrite magnetic core usually having a magnetic permeability .mu. of 5000 or so is used in a transformer, a gap (hereinafter referred simply to as gap) is formed therein to reduce the effective permeability .mu. within a range of 50 to 300.
This signifies that a gap having a great magnetic reluctance needs to exist in a ferrite magnetic core of which magnetic reluctance is originally small, whereby a greater leakage flux is generated in the periphery of the gap.
It is generally known that such leakage flux exerts at least two harmful influences as follows:
(1) Noise is induced in peripheral apparatus (components) which are prone to be affected by magnetic induction. PA0 (2) In case the coil is so wound as to surround the gap, there occurs an abnormal generation of heat in the coil around the gap due to the leakage flux.
For the purpose of solving the above problems, a variety of improvements have been developed.
In an attempt to settle the problem (1), an exemplary method has been developed for forming a gap merely in the coil alone. However, such method brings about another fault that worsens the problem (2) on the contrary.
With regard to the problem (2), some prior examples are known as disclosed in Japanese Patent Laid-open No. 55 (1980)-77115 and Utility Model Laid-open No. 57 (1982)-130402, wherein a gap positioned in a coil is divided magnetically into a plurality of serial portions so as to disperse the concentration of leakage flux. In the prior means developed for solving the problems (1) and (2), there are known examples as disclosed in Japanese Utility Model Publication Nos. 53 (1978)-53850 and 60 (1985)-7448, wherein a gap filler, of which relative permeability is greater than that of air (greater than 1), is used to reduce the magnetic reluctance in the gap portion so as to diminish the leakage flux.
When such gap filler of a material having a greater relative permeability than that of air (greater than 1) is disposed inside of a coil, there exists a possibility that the problems (1) and (2) can be solved to some extent.
However, even in such an improved structure, another problem is still left unsettled that the leakage flux is concentrated on the boundary between the gap and the magnetic core, and in addition a new problem also arises with regard to difficulty in obtaining a satisfactory material which has an adequate permeability as a gap filler while still retaining a high saturation flux density and low core loss characteristic equivalent to that of the magnetic core. Consequently, some disadvantages are unavoidable including that the coil wound on the boundary between the gap and the magnetic core is heated to an abnormal extent, and the gap portion is also heated excessively due to the core loss of the gap filler material, and further the B-H curve of the magnetic core with the gap filler inserted therein is rendered nonlinear to eventually cause wave for distortion when the coil device is used in a transformer. Thus, in the current technical stage, completely effective improvements are not available.